In order to have an exact copy of your WoW install, just copy the entire folder. If you're fine with doing a fresh install over the Internet but still want nothing to change, copy the following folders from your old install to the new install:
Errors, Fonts, Interface, Logs, Screenshots, Sound, WTF (don't worry if you don't have some of these). You'll need about
50-55GB free on whatever portable storage you're using to copy the entire folder, but only
1-2GB for the specific folders I mentioned.
Reasoning behind each folder:
- Errors - history of crashes, keep just in case support wants them
- Fonts - manually created folder to replace game fonts, you probably don't have it
- Interface - addon installs (required)
- Logs - same reason as errors, various logs including taints, chat, and combat if you have those enabled (I have a script that organizes logs into numbered files, I have the last 999 chat logs for personal use)
- Screenshots - always nice for a trip down memory lane, I wish I kept screenshots I had before the end of Wrath when I lost them
- Sound - same reason as fonts
- WTF - all game and addon settings (required)
All the other folders and files in the WoW install will automatically be created on a fresh install or when logging into the game